This proposal will extend recent data obtained in this laboratory to improve the efficacy and safety of warfarin and heparin. 1) The mechanisms whereby warfarin increases Xa inhibitory activity will be determined by a series of experiments in which the differences in the inhibition of 125I-labelled activated factor X (Xa) between plasma from normal and warfarin-treated patients will be determined. 2) The feasibility of a low-dose warfarin regimen will be investigated in rabbits. The following parameters will be used to compare the efficacy of low-dose warfarin therapy to a normal regimen: prothrombin time, Factor VII activity, Xa inhibitory activity, and inhibition of thrombosis induced by Xa and tissue thromboplastin. 3) Our laboratory has recently raised an antibody against commercial heparin. Building on this technique antibodies to heparin fractionated by affinity and gel chromatography will be prepared. These antibodies will be used to determine whether or not heparin is endogenous to human plasma and to develop both an enzyme-immunoassay using a horseradish peroxidase-heparin complex and a radio-immunoassay using 35S-heparin. Crossed immunoelectrophoresis will be employed to isolate and prepare monospecific anti-heparin antibodies that will be used to identify regions of heparin that are either involved or not involved in its catalytic activity.